The conventional types of digital tachometers can be classified into two categories, the digital display touchless (photo) type tachometers and the digital display contact type tachometers. The digital display touchless tachometer is suitable for medium and high speed measurement as well as for general purpose speed measurement. Because the speed detecting is accomplished by optical methods, there is no loading effect on this type of tachometer, and therefore provides the advantages of being more precise and safe. But due to the characteristics of the electronic circuit structure, it is not usable for surface speed measurement and very low speed measurement. On the other hand, the digital display contact tachometer is particularly suitable for low speed and surface speed measurement. However, the contact measurement is subject to extra contact load that will slightly decrease the precision of measurement and it is not suitable for measuring the high speed or the speed of an object which is impractical to make contact with. Therefore, the user has to obtain both of these two types of tachometers to accomodate the different measuring applications, thus causing extra expense and operational inconvenience.
In order to eliminate the above shortcomings, a new multipurpose combined touchless and contact type tachometer is provided. The special features of the inventive concept are to combine the touchless and contact type tachometer signal receivers into a single complete unit, and provide a main electronic circuit which is common to both systems. Since only a few extra parts are required to assemble the two different systems into a single complete unit, its costs are as low as those of the single purpose type tachometers, and the users are not required to purchase two types of tachometers. This provides a more convenient and economical system. Also because one end of this dual purpose tachometer is used for the touchless tachometer and the other end for the contact tachometer, a specially designed "display decoding circuit" is incorporated into this tachometer for rotating the display 180.degree..